Fluid mixing device

ABSTRACT

A fluid mixing device comprising at least one mixing element specifically oriented with respect to the direction of fluid flow through the device. This novel orientation of the mixing element results in improved fluid mixing.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application claims the benefit under 35 U.S.C.§119(e) of provisional patent application Ser. No. 60/209,597, filedJun. 6, 2000, the contents of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] In one of its aspects, the present invention relates to a fluidmixing device. In another of its aspects, the present invention relatesto a method for mixing a fluid.

[0004] 2. Description of the Prior Art

[0005] Mixing devices are known in the art and have been used to promotefluid turbulence—for example, to improve contact between elements in theflow path. Industrial applications of mixing are widely varied, andinclude heat exchange, reactor engineering and non-reactive blending.

[0006] One specific area of reactor engineering where mixing has beenused is in the design of fluid treatment devices, particularly fluidradiation treatment devices. A specific such fluid radiation treatmentdevice includes ultraviolet (UV) disinfection devices for water andwastewater treatment. The performance of UV disinfection devicesdepends, at least in part, on providing a prescribed dose of UVradiation to all fluid elements passing through (or otherwise beingtreated by) the device.

[0007] The UV dose received by a fluid element is defined as the productof UV intensity and exposure time. The accumulated UV dose received by afluid element exiting the device is the sum of the individual dosesreceived at each position. Since the UV intensity is attenuated with thedistance from the UV source, it is desirable to mix fluid elements fromregions far from the UV source to regions of higher intensity nearer tothe source, thereby ensuring they receive an adequate dose of UVradiation. This type of mixing is particularly desirable when thetransmittance of UV radiation through the fluid being treated is low,causing an increase in the attenuation of UV intensity with distancefrom the source—this is commonly encountered in UV disinfection devicesfor the treatment of wastewater.

[0008] U.S. Pat. No. 5,846,437 [Whitby et al. (Whitby)], assigned to theassignee of the present application, teaches turbulent mixing in a UVsystem. More specifically, Whitby teaches the use of one or morering-shaped devices (e.g., washers) at predetermined locations on theexterior surface of each lamp unit in the system and/or ring-shapeddevices upstream of each lamp unit to increase turbulent mixing of fluidpassing by the lamp units. While the use of such ring-shaped devices astaught in Whitby is useful in increasing turbulence between the lampunits, the turbulent flow of fluid tends to be of a random ornon-ordered (e.g., isotropic) nature.

[0009] In many systems, such as those where the mixing zone islongitudinal with respect to the direction of fluid flow therethrough,it is desirable to have plug flow in the axial direction and effectivemixing in the radial direction. A specific or ordered pattern of fluidflow in the mixing zone is desirable (e.g., a “particle” of fluidoscillating toward and away from the lamp as it passes longitudinallywith respect thereto), which is in contrast to general mixing in alldirections (i.e., in contrast to random mixing or turbulence taught byWhitby). A longitudinal vortex is an example of this type of flowpattern. Vortices can be formed actively through energy input to thefluid, such as by employing a motorized fluid impeller.

[0010] Another means of achieving vortex generation is through the useof a passive element which is designed to cause the formation of thedesired flow pattern (vortex generator).

[0011] U.S. Pat. Nos. 5,696,380, 5,866,910 and 5,994,705 [all in thename Cooke et al. (Cooke)] teach a flow-through photochemical reactor.The subject reactor taught by Cooke comprises an elongate annularchannel in which is disposed an elongate radiation source. The channelincludes static, fluid-dynamic elements for passively inducingsubstantial turbulent flow within the fluid as it passes through thechannel. According to Cooke, each such static, fluid-dynamic elementadvantageously creates a pair of “tip vortices” in the fluid flow pasteach element. The “tip vortices” purportedly are counter-rotating aboutan axis parallel to the elongate annular chamber.

[0012] U.S. Pat. No. 6,015,229 [Cormack et al. (Cormack)], assigned tothe assignee of the present application, teaches a fluid mixing device.The fluid mixing device comprises a series of “delta wing” mixingelements which cause the formation of vortices thereby improving fluidmixing. A specific embodiment of such a device illustrated in Cormack isthe use of “delta wing” mixing elements to cause such vortex mixingbetween UV radiation sources in an array of such sources. This createsthe potential for increasing distance between adjacent UV radiationsources in the array which, in turn, allows for a reduction in hydraulichead loss of the fluid flow through a UV disinfection system comprisingthe fluid mixing device.

[0013] Despite the advances in the art made by Cooke and Cormack, thereis still room for improvement. For example, there is an ongoing need forfluid mixing devices which, when used in fluid treatment devices such asUV disinfection systems, are capable of improving the UV radiation doseequivalent (this term will be described in more detail hereinbelow)delivered by the disinfection system to the fluid being treated.

[0014] Further, the vortex generating devices taught by Cooke andCormack use the kinetic energy of the flowing fluid to cause mixing.This necessarily results in a loss of fluid pressure, or fluid head inan open channel system. This is very undesirable in UV disinfectionsystems, particularly those used to treat municipal wastewater, sincethe UV disinfection system typically is the last station of themulti-station treatment plant. As such, wastewater entering thetreatment plant typically suffers hydraulic head loss as it passes fromstation to station with the result that, when the wastewater reaches theUV disinfection system, there is not much room for further significantloss of hydraulic head. Accordingly, it would be highly desirable tohave a fluid mixing device which, in addition to improving doseequivalent as described above when used in a UV disinfection system,resulted in reducing the hydraulic head loss of fluid being treated bythe UV disinfection system.

SUMMARY OF THE INVENTION

[0015] It is an object of the invention to provide a novel fluid mixingdevice which obviates or mitigates at least one of the above-mentioneddisadvantages of the prior art.

[0016] It is an object of the invention to provide a novel method formixing a fluid which obviates or mitigates at least one of theabove-mentioned disadvantages of the prior art.

[0017] It is an object of the invention to provide a novel fluidtreatment system comprising a fluid mixing device which obviates ormitigates at least one of the above-mentioned disadvantages of the priorart.

[0018] Accordingly, in one of its aspects, the present inventionprovides a fluid mixing device for mixing a fluid having a direction offluid flow, the device comprising at least one mixing element to createat least one vortex adjacent to a surface downstream of the mixingelement, the mixing element having a first normal located at a centroidthereof and the surface having a second normal which intersects thefirst normal at the centroid, wherein the first normal, the secondnormal and the direction of fluid flow are in a non-planar relationship.

[0019] In another of its aspects, the present invention provides a fluidmixing device comprising at least one mixing element for mixing a flowof fluid having a direction of fluid flow, the at least one mixingelement comprising a surface having a first normal which is:

[0020] (i) acutely angled with respect to a first plane having a secondnormal substantially perpendicular to the direction of fluid flow; and

[0021] (ii) acutely angled with respect to a second plane parallel tothe direction of fluid flow and orthogonal to the first plane.

[0022] In yet another of its aspects, the present invention provides afluid mixing device comprising at least one mixing element for mixing aflow of fluid having a direction of fluid flow, the at least one mixingelement comprising a surface having a normal which is acutely angledwith respect to each of two planes which are orthogonal to one another,each plane intersecting on a line substantially parallel to thedirection of fluid flow.

[0023] In yet another of its aspects, the present invention provides afluid mixing device comprising at least one mixing element for mixing aflow of fluid having a direction of fluid flow, the at least one mixingelement comprising a surface having a normal which is acutely angledwith respect to a first plane and a second plane which is orthogonal tothe first plane, the first plane and the second plane intersecting on aline substantially parallel to the direction of fluid flow.

[0024] In yet another of its aspects, the present invention provides afluid mixing device for mixing a fluid having a direction of fluid flow,the device comprising at least one mixing element to create at least onevortex adjacent to a surface downstream of the mixing element, themixing element oriented in a manner such that a single rotation aroundits nearest edge to the surface causes the mixing element to becomeparallel to a tangent to the surface at a point nearest to the mixingelement, describing an axis of rotation that is oblique with respect tothe direction of fluid flow.

[0025] In yet another of its aspects, the present invention provides amethod for mixing a fluid having a direction of fluid flow, the methodcomprising the steps of:

[0026] (i) disposing the at least one mixing element in the fluid flow;and

[0027] (ii) positioning the at least one mixing element to create atleast one vortex adjacent to a surface downstream of the mixing elementsuch that the mixing element has a first normal located at a centroidthereof and the surface has a second normal which intersects the firstnormal at the centroid,

[0028] wherein the first normal, the second normal and the direction offluid flow are in a non-planar relationship.

[0029] In yet another of its aspects, the present invention provides amethod for mixing a fluid having a direction of fluid flow, the methodcomprising the steps of:

[0030] (i) disposing the at least one mixing element in the fluid flow;and

[0031] (ii) positioning the at least one mixing element such that asurface thereof has a normal which is acutely angled with respect to afirst plane and a second plane which is orthogonal to the first plane,the first plane and the second plane each having an axis of rotationsubstantially parallel to the direction of fluid flow.

[0032] Thus, the present inventors have designed a novel fluid devicehaving at least one mixing element oriented in a manner to achieveimproved mixing of the fluid.

[0033] To understand the novel structure of one embodiment of thepresent fluid mixing device, it is appropriate to consider a fluidmixing device for mixing a fluid having a direction of fluid flow, thedevice comprising at least one mixing element. The mixing element isdesigned to create at least one vortex adjacent to a surface the mixingdevice which is downstream of the mixing element. The mixing elementcomprises a centroid. As used throughout this specification, the term“centroid” is intended to mean the position or point on the mixingelement corresponding its center of its mass, the point at which themixing element would be stable, or balance, under the influence ofgravity—the is also conventionally referred to as the center of gravity.The mixing element is oriented in the fluid flow in a manner such that afirst normal located at the centroid of the mixing element intersects asecond normal emanating from the surface at the centroid of the mixingelement such that the first normal, the second normal and the directionof fluid flow are in a non-planar relationship—see FIG. 2. In contrast,Cooke and Cormack teach a mixing element disposed in the fluid flow in amanner such that the first normal, the second normal and the directionof fluid flow are in a planar relationship—see FIG. 1.

[0034] This novel orientation of the mixing element results in improvedfluid mixing. When the fluid mixing device is employed in a fluidtreatment system such as UV disinfection system the improved fluidmixing is manifested in an improvement of UV dose delivery of thesystem. Additionally, in various preferred embodiments of the presentfluid mixing device, such improved fluid mixing is accompanied by areduction in hydraulic head loss of fluid passing through the system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Embodiments of the present invention will be described withreference to the accompanying drawings, wherein like numerals designatelike elements, and in which:

[0036]FIG. 1 illustrates a Cormack-type mixing element pointeddownstream to the direction of fluid flow;

[0037]FIG. 2 illustrates an embodiment of the present fluid mixingdevice wherein the mixing element is pointed upstream with respect tothe direction of fluid flow

[0038]FIG. 3a illustrates an isometric view of a schematicrepresentation of disposition of a mixing element in the present fluidmixing device in a co-ordinate system defined by two orthogonal planes;

[0039]FIG. 3b illustrates a top view of FIG. 3a;

[0040]FIG. 3c illustrates a front view of FIG. 3a;

[0041]FIG. 3d illustrates a side view of FIG. 3a;

[0042]FIG. 4 illustrates a Cormack-type mixing element pointeddownstream to the direction of fluid flow;

[0043] FIGS. 5-8 illustrate various embodiments of a Cormack-type mixingelement pointed downstream to the direction of fluid flow;

[0044]FIG. 9 illustrates a Cormack-type mixing element pointed upstreamto the direction of fluid flow;

[0045] FIGS. 10-11 illustrate various embodiments of the present fluidmixing device wherein the mixing element is pointed upstream withrespect to the direction of fluid flow;

[0046]FIG. 12 illustrates a further embodiment of the present fluidmixing device wherein the mixing element is pointed downstream withrespect to the direction of fluid flow;

[0047] FIGS. 13-16 illustrate further embodiments of the present fluidmixing device;

[0048]FIG. 17 shows a preferred embodiment of the fluid mixing device ofFIG. 12 applied to a UV disinfection module for the treatment of waterand wastewater in an open channel system;

[0049]FIGS. 18a, 18 b and 18 c illustrate a preferred embodiment of thefluid mixing device of FIG. 14 applied to a UV disinfection module forthe treatment of water and wastewater in a fluid treatment systemcomprising a closed fluid irradiation zone;

[0050]FIG. 19 shows relative mixing efficiency and fluid energy loss ofthe fluid mixing devices illustrated in FIGS. 4-8 (mixing efficiency ismeasured by UV Dose Equivalent (mW-s/cm²) and fluid energy loss by HeadLoss (Pa); and

[0051]FIG. 20 shows relative mixing efficiency and fluid energy loss ofthe fluid mixing devices illustrated in FIGS. 9-11 (mixing efficiency ismeasured by UV Dose Equivalent (mW-s/cm²) and fluid energy loss by HeadLoss (Pa).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0052] Throughout this specification, reference will be made toemployment of the present fluid mixing device in fluid treatment systemssuch as fluid radiation treatment systems (e.g., UV disinfectionsystems). Those of skill in the art will recognize that, while thepreferred embodiment of the present invention is a fluid radiationtreatment system incorporating the present fluid mixing device, thescope of the invention is not to be so limited. Specifically, it isenvisioned that the present fluid mixing device will have manyapplications outside the art of fluid radiation treatment systems.

[0053] As used throughout this specification, repeated reference is madeto the term “normal”. In connection with a point on a planar surface,the term “normal” is intended to mean a line perpendicular to thetangent plane at that point on the surface. In connection with a pointon a curved surface, the term “normal” is intended to mean a lineperpendicular to the tangent plane at that point on the surface.

[0054] With reference to FIGS. 3a-3 d, an explanation will be providedon orientation of the mixing element(s) in certain embodiments of thepresent fluid mixing device. This explanation should be read in thecontext of the non-planar relationship of the first normal, the secondnormal and the direction of fluid flow described above. This orientationis exemplified in a number of different embodiments which will bedescribed hereinbelow.

[0055] As shown in FIGS. 3a-3 d, a co-ordinate system is defined by twofixed planes X,Y. Planes X,Y intersect at an axis (or at a line) whichis common to the direction of fluid flow through the fluid mixingdevice.

[0056] As shown, a pair of fixed mixing elements 10,12 are disposed onthe co-ordinate system defined by planes X,Y. Mixing elements 10,12converge to an apex region 14 which is oriented in a direction oppositeto the direction of fluid flow (this is a preferred embodiment only andthe orientation of the apex could be reversed if desired).

[0057] With reference to FIG. 3b, a normal 16 to mixing element 10 hasbeen illustrated. With further reference to FIG. 3b, it will be apparentthat normal 16 forms an acute angle B with respect plane Y. Withreference to FIG. 3c, it will be seen that normal 16 forms an acuteangle A with respect to plane X. Thus, normal 16 forms independent acuteangles to each of planes X,Y.

[0058] In the discussion which follows with respect to the embodimentsillustrated in FIGS. 5-8 and 10-16, the feature of the non-planarrelationship of the first normal, the second normal and the direction offluid flow described above is exemplified.

[0059] As will be appreciated by those of skill in the art, there aremany embodiments which may be conceived within the scope of the presentfluid mixing device. A number of these will be discussed below however,those of skill in the art will readily recognize that many otherembodiments are possible. Further, while some details of attachment ofthe mixing elements to other elements in the mixing device are provided(see, for example, FIGS. 17 and 18), the precise details will vary fromapplication to application and are within the purview of person skilledin the art—see, for example, Cormack.

[0060] In a preferred embodiment of the present fluid mixing device, apair of mixing elements is utilized. A pair of mixing surfaces allowseach surface to be positioned independently while still generating pairsof counter-rotating vortices. This means that splitting a single surfacegenerating two vortices into a pair of surfaces each generating a vortexincreases flexibility in the overall orientation of the surfaces, andpermits optimization that is otherwise difficult to achieve.

[0061] Orienting (e.g., by angling) each surface with respect to twoplanes as described above causes the projected area of the surface inthe direction of fluid flow to decrease with minimal change to theactual surface area. A reduction in projected area is desirable becausethe projected area in the direction of fluid flow is related to thefluid energy loss created by the surface. By virtue of the change inshape presented to the fluid flow, mixing performance can also beincreased.

[0062] Mixer performance may also be improved by increasing the lengthof the leading edge. One possible means of accomplishing this is throughcurvature of the edge. This increases edge length for a given projectedarea of the surface in the direction of fluid flow. The total edgelength available at a given position may be adjusted by selecting anappropriate curvature. This means that the strength of the vortex neednot be constant with radial position. This can be particularlyadvantageous in UV disinfection systems, where it is desirable to mixfluid elements far from the UV to positions near the source. The samelogic can be applied to curvature of the face of the mixing surface.This can increase the actual surface area for a given projected area,and the degree of curvature may be similarly chosen so as to alter thestrength of mixing as a function of radial position. Both types ofcurvature can be used alone or in combination to increase the overallmixing effectiveness of a vortex generating static mixing device.

[0063] By creating a passage for fluid flow between the pair of mixingsurfaces, disinfection performance can be improved. The degree ofimprovement depends, at least in part, on the size of the opening andthe position of the surfaces. The open passage may be created either byremoving surface area from each surface, or preferably, by repositioningeach surface so as to preserve overall actual or effective area. Ifsurface area is removed, the net effect of an open passage for fluidflow can be to reduce disinfection performance and fluid energy loss.The open passage for fluid flow can only be accomplished with a pair ofsurfaces. The trailing edge created by separating the surfaces may alsobe curved so as to minimize the formation of vortices which coulddestabilize the main vortex created by the leading edge. Open passagesfor fluid flow and curved edges have the added advantage of beingrelatively resistant to debris fouling in systems where this is aconcern, such as in UV disinfection systems for the treatment ofwastewater.

[0064] Generating pairs of vortices with a pair of surfaces increasesoverall flexibility of a static mixing device by allowing theoptimization of mixing effectiveness and fluid energy loss. Angling themixing surfaces to achieve the non-planar relationship of the firstnormal, the second normal and the direction of fluid flow as describedabove, can decrease overall fluid energy loss and increase mixingeffectiveness. Applying curvature to the edges or faces of the mixingsurface also increases mixing performance and, in some cases, decreasesoverall fluid energy loss. Additional increase in mixing performance orreduction in fluid energy loss can be achieved by creating a passage forfluid flow between surfaces. The combination of these elements to createdevices which are optimized with respect to both mixing effectivenessand overall fluid energy loss is an advantage of the present mixingdevice.

[0065] One tool used to visualize fluid flow patterns is computationalfluid dynamics (CFD). CFD is the analysis of fluid flow systems usingcomputer based simulations, and is widely used to solve heat and masstransfer problems. It is commonly employed in such diverse areas asaerodynamics, chemical process design, and environmental engineering.CFD is a numerical model of the Navier-Stokes equations describing fluidbehaviour. The geometry of interest is sub-divided by a “mesh” ofindividual nodes at which the fluid flow interactions are solved throughan iterative routine. An appropriate turbulence model can also beapplied. Using CFD, the fluid flow patterns can be visualized, givingpowerful insight into the conditions inside the flow geometry ofinterest. This information can be used to calculate differences in fluidbehaviour caused by addition of various mixing surfaces, reducing designtime as compared with a strictly experimental approach.

[0066] CFD is a powerful tool to compare mixers under identicalconditions, without introducing a large number of experimentalvariables. A variety of mixers were tested using CFD to determine theimpact of changes to design features such as projected area, actualarea, edge length, space between pairs of mixing surfaces, etc. ondisinfection performance and fluid energy loss. CFD can be used togenerate the particle tracks of a finite number of neutrally buoyant,massless “particles”. To simulate UV disinfection performance, aradiation model can be applied to generate fields of UV intensitiesthroughout the reactor geometry. A post processing algorithm then takesthe “particle” paths and integrates them with the intensity field todetermine the amount (or “dose”) of UV received by each fluid element.The histogram of UV doses is then analyzed using known kinetic modelsfor microbial disinfection to arrive at an overall inactivation for thereactor. This overall inactivation is reported as the “dose equivalent”for the reactor; the dose that would have to be received by each fluidelement passing through the reactor in order to achieve the calculatedoverall inactivation. This computational model, called CoDiM, isexplained in more detail in Buffle et al., “UV Reactor Conceptualizationand Performance Optimization With Computational Modeling.”, WaterEnvironment Federation, Mar. 15-18, 2000, New Orleans. See, also, Wrightet al., “An Assessment of the Bioassay Concept for UV ReactorValidation.”, Water Environment Federation, Mar. 15-18, 2000, NewOrleans.

[0067] Comparing the disinfection performance of various mixing devicescan be achieved by modelling them in the flow using CoDiM. The CoDiMpackage automatically determines the fluid energy loss, often reportedas “pressure drop” or “head loss” for a given geometry. By choosing thesame reactor geometry, energy input and flow rate, comparativeperformance of mixers can be fairly assessed. The mesh that is used bythe CoDiM to calculate the flow field should be generated in the sameway for each mixer design in order to compare results to minimizecomputationally introduced variability. Since changes to mixer designfeatures often yield improvements in performance at the expense ofenergy loss, and vice versa, it is desirable to assess both factors atthe same time. By plotting disinfection performance (“dose equivalent”)and fluid energy loss (“head loss”) on the same graph for each mixerdesign, assessed under the same conditions, a fair comparison of thedesigns can be made.

[0068] A virtual CFD model was constructed in order to compare theimpact of changes to mixer features on dose equivalent and head lossunder a consistent set of conditions. The CFD package used was Fluentv.5.0 (Lebanon, N.H.). A rectangular reactor 0.127 m×0.127 m×1.778 m(5″×5″×70″) was designed in CoDiM. A single tubular UV radiation sourcesleeve was placed in the center of the reactor and acted as the supportmember for mixers. The UV radiation source had a radius of 0.023 m(0.89″) and a length of 1.575 m (62″). The flow direction was parallelto the UV radiation source, and the inlet and outlet of the reactor weredescribed as being in-line with the direction of flow. A bulk fluidvelocity of 0.9 m/s was selected for all mixers, and water was chosen asthe fluid. The CoDiM software then calculated the fluid flow field,providing outputs for pressure drop and particle tracks for microbespassing through the reactor.

[0069] In order to assess mixer performance, disinfection modellingusing CoDiM was conducted with the particle tracks. A lamp arc length of1.47 m (58″) was used, and a lamp output of 90 W/m of germicidal UVCenergy was selected. The fluid transmittance of UV light was selected at60%. CoDiM then generated a dose histogram of the microbes passingthrough the reactor. The dose histogram is then used to generate thepercentage of inactivation of incoming microbes. A first order kineticmodel can then be applied to determine the required UV dose to achievethe required level of inactivation. In order to allow comparison of thedose equivalents for each mixer, a first order kinetic constant formicrobe inactivation of 0.53 was used for all studies. Since the sameset of conditions was used for all experiments, the dose equivalent canbe used to indicate relative mixer efficiency, and allows comparisons ofmixers to be made.

[0070] The basic mixer shape shown in FIG. 4 was used as a basis for allcomparisons. The various mixer features were then added to this shape,preserving the original angle with respect to the UV radiation source.This approach of building upon the original shape allows comparisons ofthe impact of mixer features on dose equivalent and head loss as theyare added individually. The additional features of each mixer thatdifferentiate it from FIG. 4 are described hereinbelow in Table 1.

[0071] The angle between the mixing surface and the axis of flow is heldconstant at 28° for all embodiments in the noted Figures. Table 1 refersto a single mixing surface of each pair. FIGS. 4 and 9 are considered tobe a pair of mixing surfaces butted together. TABLE 1 Figure Actual AreaProjected Area Leading Edge Number (m² × 10⁻⁴) (m² × 10⁻⁴) Length (m) 47.75 3.75 0.117 5 7.75 3.75 0.117 6 7.75 3.75 0.117 7 15.75 8.25 0.119 89.75 4.50 0.117 9 7.75 3.75 0.117 10 15.75 7.00 0.117 11 19.50 8.250.119

[0072]FIG. 4 shows a three sided mixing element angled with respect tothe main direction of fluid flow only. In this particular figure, themain direction of fluid flow is parallel to the support member of themixing surface. This support member could be a UV radiation source, suchas are used in the disinfection of water or wastewater, or it could beany other member which is parallel to the direction of fluid flow.Another way of describing the orientation of the this surface is thenon-planar relationship of the first normal, the second normal and thedirection of fluid flow described above.

[0073] This particular mixing surface creates a pair of vortices fromeach of the two leading edges that rotate in opposite directions. It ispossible to consider this mixing surface as a pair of surfaces formingtwo right triangles that are attached along one edge. By separatingthese two triangles, additional flexibility is gained in terms of angleand position that can be used to improve mixing performance and fluidenergy loss.

[0074] With reference to FIG. 5, a pair of mixing elements isillustrated. Each mixing element includes a surface shown having anormal which is acutely angled with respect to both planes in theco-ordinate system illustrated in FIG. 3.

[0075] The pair of mixing surfaces shown in FIG. 6 is similar inorientation to those of FIG. 5, but a passage for fluid flow has beencreated between surfaces. In this case, the means of creating a passagefor fluid flow is by separating each surface of the pair, keeping actualand projected surface area constant. This separation of surfaces can beachieved by repositioning each surface. An alternative passage for fluidflow can be created by simply removing material from each mixingsurface, resulting in a decrease of both effective surface area andprojected area.

[0076] The pair of mixing surfaces shown in FIG. 7 is similar to thoseof FIG. 5. In FIG. 7 the mixing elements have curved leading edges.These curved leading edges increase projected area and actual surfacearea, and increase total mixing edge length while preserving acuteangles with respect to each of two orthogonal planes, each plane havingan axis parallel to the direction of fluid flow.

[0077]FIG. 8 shows a pair of mixing surfaces similar to those of FIG. 5.In FIG. 8, the mixing surfaces themselves are curved and include curvedleading and trailing edges.

[0078] Further adjustment of angles, curves, opening sizes, etc. of theembodiments illustrated in FIGS. 5-8 can be designed to arrive at anoptimized mixing surface for a given set of conditions, such as supportsize, spacing between adjacent supports, and desired degree ofdisinfection performance vs. head loss.

[0079] With reference to FIG. 19, it can be seen that the mixingelements in FIG. 5 result in improved disinfection performance ascompared with the mixing elements of FIG. 4, together with a reductionof fluid energy loss. The increase in disinfection performanceaccompanied by a reduction fluid energy loss is an unexpected result,since the projected area, actual area and edge length remain constant.This is by virtue of the non-planar relationship of the first normal,the second normal and the direction of fluid flow described above

[0080]FIG. 19 illustrates that an open passage for fluid flow betweensurfaces as illustrated in FIG. 6 increases disinfection performancewith a concurrent reduction in head loss compared with the results forthe mixing device illustrated in FIG. 4. The open passage has thefurther benefit of reducing the tendency for debris fouling, which cansignificantly impact mixer performance in high fouling applications,such as in wastewater disinfection.

[0081]FIG. 19 further illustrates that the embodiment of FIG. 7significantly improves disinfection performance compared with theembodiment of FIG. 4. Although hydraulic head loss increased with theembodiment of FIG. 7, the fluid mixing device is useful in applicationswere enhanced disinfection performance is more important than minimizinghydraulic head loss, and in combination with other embodiments discussedherein.

[0082] With further reference to FIG. 19, it can be deduced that, byapplying curved surfaces to the mixing elements, as in FIG. 8, projectedand actual area increase while edge length remains nearly constant. Thiscombination creates additional mixing, which translates to improvementsin disinfection performance.

[0083]FIG. 9 illustrates mixing elements which are of similar shape andorientation to the surface shown in FIG. 4, but reversed with respect tothe direction of flow.

[0084] Various features of the mixing elements illustrated in FIGS. 5-8were selected and combined to arrive at the mixing elements in FIGS. 10and 11.

[0085] As can be seen with reference to FIG. 20, an improvement indisinfection performance is achieved in the fluid mixing devices ofFIGS. 10 and 11, compared with that of FIG. 9. Although hydraulic headloss increased with the embodiments of FIGS. 10 and 11, the fluid mixingdevices are useful in applications were enhanced disinfectionperformance is more important than minimizing hydraulic head loss, andin combination with other embodiments discussed herein. The fluid mixingdevices of FIGS. 10 and 11 delivered the greatest dose equivalent of anyof the mixing devices tested. In the embodiment shown in FIG. 10, theopen passage for fluid flow between mixing surfaces was created byremoving actual surface area from each mixer, resulting in a decrease ofdisinfection performance and head loss.

[0086] FIGS. 12-16 illustrate further fluid mixing devices fallingwithin the scope of the present invention.

[0087]FIG. 12 shows an embodiment four “fin-shaped” mixing surfaces.Each mixing surface is three-sided and generally curved. For a givenpair of theses mixing surfaces, a generally lens shaped opening isdefined by opposed concaved-shaped sides of the two mixing surfaces.

[0088]FIG. 13 shows an embodiment of pairs of four sided mixingsurfaces. The faces of the surfaces are curved such that the surfacesare always normal to the support surface at each point in thelongitudinal direction. In UV disinfection of water, this providesminimal shadowing of the water being treated, improving overalldisinfection performance. The curvature was chosen in this system suchthat the edges are straight, but they could also be curved. The mixerembodiment shown in this Figure has open passages for fluid flow betweenpairs of fins.

[0089] The mixer shown in FIG. 14 is similar in most respects to themixing device illustrated in FIG. 13 except for the omission of theopenings between adjacent fins for fluid flow.

[0090]FIG. 15 shows another embodiment of pairs of four sided mixingsurfaces with curved edges and faces.

[0091] The principles described above can be applied in the creation ofadditional four sided surfaces, other than those shown in thesepreferred embodiments. The number of pairs of mixing surfaces mounted toa given support is chosen based upon the geometry of adjacent supportmembers.

[0092]FIG. 16 illustrates a number of pairs of three sided mixingsurfaces with curved edges and no spaces between pairs of mixingsurfaces. Additional optimization of this mixer embodiment is possibleby selective application of the principles previously disclosed.

[0093]FIG. 17 shows a preferred embodiment of a mixer mounted to a UVdisinfection module. Details on the construction of such a module may befound in the following documents:

[0094] U.S. Pat. No. 4,482,809 [Maarschalkerweerd];

[0095] U.S. Pat. No. 4,872,980 [Maarschalkerweerd];

[0096] U.S. Pat. No. 5,006,244 [Maarschalkerweerd];

[0097] International publication number WO 00/51943 [Traubenberg et al.(Traubenberg)]; and

[0098] International publication number WO 00/26144 [Pearcey et al.(Pearcey)].

[0099] The mixer elements are similar to the ones illustrated in FIG.12. The direction of flow is parallel to the radiation sources,preferably from left to right in FIG. 17. A given pair of mixers issplit in half in order to be mounted to the radiation sources fromeither side. This method of attachment allows the mixer to be easilyadded to the UV disinfection module after manufacturing. The position ofthe mixing surfaces is chosen such that it is mounted as close aspossible to the UV radiation source surface, upstream of the radiationsources.

[0100]FIGS. 18a, 18 b and 18 c illustrated embodiments of a mixingdevice mounted to another type of UV disinfection module. Details on theconstruction of such a device may be found in Cormack referenced aboveand the following documents:

[0101] U.S. Pat. No. 5,418,370 [Maarschalkerweerd];

[0102] U.S. Pat. No. 5,539,210 [Maarschalkerweerd]; and

[0103] U.S. Pat. No. Re36,896 [Maarschalkerweerd].

[0104] The mixer elements included in this embodiment is that of FIG.14. Since the mixing elements are preferably mounted upstream of theradiation sources in order to produce the desired effects ondisinfection performance, mixing elements may be mounted to differentparts of the module, depending on whether the radiation source supportmembers are located upstream or downstream. FIG. 18a illustrates adouble module (i.e., upstream and downstream modules) fluid treatmentsystem and illustrates the position of mixing elements in relation tothe direction of the flow of fluid. FIG. 18b illustrates a single module(i.e., upstream module) fluid treatment system. As an alternateembodiment, FIG. 18c illustrates mount of the mixing elements to thehousing of the fluid treatment system rather than to the module.

[0105] While this invention has been described with reference toillustrative embodiments and examples, the description is not intendedto be construed in a limiting sense. Thus, various modifications of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thisdescription. It is therefore contemplated that the appended claims willcover any such modifications or embodiments.

[0106] All publications, patents and patent applications referred toherein are incorporated by reference in their entirety to the sameextent as if each individual publication, patent or patent applicationwas specifically and individually indicated to be incorporated byreference in its entirety.

What is claimed is:
 1. A fluid mixing device for mixing a fluid having adirection of fluid flow, the device comprising at least one mixingelement to create at least one vortex adjacent to a surface downstreamof the mixing element, the mixing element having a first normal locatedat a centroid thereof and the surface having a second normal whichintersects the first normal at the centroid, wherein the first normal,the second normal and the direction of fluid flow are in a non-planarrelationship.
 2. The fluid mixing device defined in claim 1, wherein thesurface comprises a leading edge.
 3. The fluid mixing device defined inclaim 1, wherein the surface comprises a trailing edge.
 4. The fluidmixing device defined in claim 1, wherein the surface comprises aleading edge and a trailing edge.
 5. The fluid mixing device defined inclaim 4, wherein the leading edge and trailing edge are substantiallyparallel.
 6. The fluid mixing device defined in claim 5, wherein theleading edge and the trailing edge are interconnected by a wing tipedge.
 7. The fluid mixing device defined in claim 6, wherein the wingtip edge comprises an edge substantially parallel to the direction offluid flow.
 8. The fluid mixing device defined in claim 4, wherein the aleading edge and a trailing edge are non-parallel.
 9. The fluid mixingdevice defined in claim 8, wherein the one of leading edge and thetrailing edge is substantially perpendicular to the direction of fluidflow.
 10. The fluid mixing device defined in claim 2, wherein leadingedge comprises a substantially curved edge.
 11. The fluid mixing devicedefined in claim 2, wherein leading edge comprises a substantiallystraight edge.
 12. The fluid mixing device defined in claim 2, whereintrailing edge comprises a substantially curved edge.
 13. The fluidmixing device defined in claim 2, wherein trailing edge comprises asubstantially straight edge.
 14. The fluid mixing device defined inclaim 1, wherein the mixing element comprises a planar surface.
 15. Thefluid mixing device defined in claim 1, wherein the mixing elementcomprises a curved surface.
 16. The fluid mixing device defined in claim1, wherein an the mixing element comprises an apex portion.
 17. Thefluid mixing device defined in claim 16, wherein the apex portion isoriented to point substantially upstream with respect to the directionof fluid flow.
 18. The fluid mixing device defined in claim 16, whereinthe apex portion is oriented to point substantially downstream withrespect to the direction of fluid flow.
 19. The fluid mixing devicedefined in claim 1, comprising a first mixing element and a secondelement.
 20. The fluid mixing device defined in claim 19, wherein thefirst mixing element and the second mixing element are substantiallymirror images of one another about the first plane or the second plane.21. The fluid mixing device defined in claim 19, wherein the firstmixing element and the second mixing element are substantiallynon-mirror images of one another about the first plane or the secondplane.
 22. The fluid mixing device defined in claim 19, wherein thefirst mixing element comprising a first leading edge and a firsttrailing edge.
 23. The fluid mixing device defined in claim 19, whereinthe second mixing element comprising a second leading edge and a secondtrailing edge.
 24. The fluid mixing device defined in claim 19, whereinthe first mixing element comprising a first leading edge and a firsttrailing edge, and the second mixing element comprising a second leadingedge and a second trailing edge.
 25. The fluid mixing device defined inclaim 22, wherein at least one of the first leading edge and the secondleading edge comprise a substantially straight edge.
 26. The fluidmixing device defined in claim 22, wherein both of the first leadingedge and the second leading edge comprise a substantially straight edge.27. The fluid mixing device defined in claim 22, wherein at least one ofthe first leading edge and the second leading edge comprise asubstantially curved edge.
 28. The fluid mixing device defined in claim22, wherein both of the first leading edge and the second leading edgecomprise a substantially curved edge.
 29. The fluid mixing devicedefined in claim 22, wherein the first trailing edge and the secondtrailing edge are integral such that the first mixing element and thesecond mixing element are interconnected.
 30. The fluid mixing devicedefined in claim 22, wherein the first trailing edge and the secondtrailing edge are in spaced relation to define an opening between thefirst mixing element and the second mixing element.
 31. The fluid mixingdevice defined in claim 22, wherein the first leading edge and thesecond leading edge are integral such that the first mixing element andthe second mixing element are interconnected.
 32. The fluid mixingdevice defined in claim 19, wherein the first mixing element comprises afirst apex portion.
 33. The fluid mixing device defined in claim 19,wherein the second mixing element comprises a second apex portion. 34.The fluid mixing device defined in claim 19, wherein the first mixingelement comprises a first apex portion and the second mixing elementcomprises a second apex portion.
 35. The fluid mixing device defined inclaim 32, wherein the first apex portion is oriented substantiallydownstream with respect to the direction of fluid flow.
 36. The fluidmixing device defined in claim 32, wherein the second apex portion isoriented substantially downstream with respect to the direction of fluidflow.
 37. The fluid mixing device defined in claim 32, wherein the firstapex portion and the second apex portion are oriented substantiallydownstream with respect to the direction of fluid flow.
 38. The fluidmixing device defined in claim 32, wherein the first apex portion isoriented substantially upstream with respect to the direction of fluidflow.
 39. The fluid mixing device defined in claim 32, wherein thesecond apex portion is oriented substantially upstream with respect tothe direction of fluid flow.
 40. The fluid mixing device defined inclaim 32, wherein the first apex portion and the second apex portion areoriented substantially upstream with respect to the direction of fluidflow.
 41. The fluid mixing device defined in claim 1, wherein the atleast one mixing element comprises a plane.
 42. The fluid mixing devicedefined in claim 1, wherein the at least one mixing element comprises awedge.
 43. A fluid mixing device comprising at least one mixing elementfor mixing a flow of fluid having a direction of fluid flow, the atleast one mixing element comprising a surface having a first normalwhich is: (i) acutely angled with respect to a first plane having asecond normal substantially perpendicular to the direction of fluidflow; and (ii) acutely angled with respect to a second plane parallel tothe direction of fluid flow and orthogonal to the first plane.
 44. Afluid mixing device comprising at least one mixing element for mixing aflow of fluid having a direction of fluid flow, the at least one mixingelement comprising a surface having a normal which is acutely angledwith respect to each of two planes which are orthogonal to one another,each plane intersecting on a line substantially parallel to thedirection of fluid flow.
 45. A fluid mixing device comprising at leastone mixing element for mixing a flow of fluid having a direction offluid flow, the at least one mixing element comprising a surface havinga normal which is acutely angled with respect to a first plane and asecond plane which is orthogonal to the first plane, the first plane andthe second plane intersecting on a line substantially parallel to thethe direction of fluid flow.
 46. A fluid mixing device for mixing afluid having a direction of fluid flow, the device comprising at leastone mixing element to create at least one vortex adjacent to a surfacedownstream of the mixing element, the mixing element oriented in amanner such that a single rotation around its nearest edge to thesurface causes the mixing element to become parallel to a tangent to thesurface at a point nearest to the mixing element, describing an axis ofrotation that is oblique with respect to the direction of fluid flow.47. A radiation source module comprising the fluid mixing device definedin claim
 1. 48. A fluid treatment system comprising the fluid mixingdevice defined in claim
 1. 49. A fluid radiation treatment systemcomprising the fluid mixing device defined in claim
 1. 50. A method formixing a fluid having a direction of fluid flow, the method comprisingthe steps of: (i) disposing the at least one mixing element in the fluidflow; and (ii) positioning the at least one mixing element to create atleast one vortex adjacent to a surface downstream of the mixing elementsuch that the mixing element has a first normal located at a centroidthereof and the surface has a second normal which intersects the firstnormal at the centroid, wherein the first normal, the second normal andthe direction of fluid flow are in a non-planar relationship.
 51. Amethod for mixing a fluid having a direction of fluid flow, the methodcomprising the steps of: (i) disposing the at least one mixing elementin the fluid flow; and (ii) positioning the at least one mixing elementsuch that a surface thereof has a normal which is acutely angled withrespect to a first plane and a second plane which is orthogonal to thefirst plane, the first plane and the second plane each having an axis ofrotation substantially parallel to the direction of fluid flow.